Recording materials using a combination of an electron-donating dye precursor (hereinafter referred to as color former) and an electron-accepting compound (hereinafter referred to as color developer) are well known as a pressure-sensitive paper, a heat-sensitive paper, a light- and pressure-sensitive paper, an electric heat-sensitive paper, and the like. Details of these recording materials are described, e.g., in British Patent No. 2,140,449, U.S. Pat. Nos. 4,480,052 and 4,436,920, Japanese Patent Publication No. 23922/85, and Japanese Patent Application (OPI) Nos. 179836/82, 123556/85, and 123557/85 (The term "OPI" as used herein means "unexamined published Japanese patent application").
These recording materials must (1) provide an image having sufficient color density with sufficient color formation sensitivity, (2) cause no fog, (3) provide an image having sufficient fastness, (4) form a hue suitable for copying machines, (5) have a high S/N ratio, (6) provide an image sufficiently resistant to chemicals, and the like. However, none of the conventional recording materials has completely fulfilled these requirements.
While, in particular, heat-sensitive recording materials have recently undergone a marked development, they still have disadvantages of fog generation due to contact with solvents, etc. and decoloration or discoloration of a recorded image due to contact with fats and oils, chemicals, fingers, etc. Specifically, if a heat-sensitive recording material comes into contact with stationery, such as aqueous or oily inks, fluorescent inks, stamp inks, adhesives, starch paste, diazo developers, etc., or cosmetics, such as hand creams, emulsions, etc., the white background tends to fog or the color developed area tends to discolor. Further, heat-sensitive materials recently enjoy an increasing demand as POS (Point-of-sales) labels. Accordingly, there is a great and growing demand in the market for chemically resistant heat-sensitive materials including heat-sensitive labels.
Furthermore, with the recent increase in use of bar codes, etc. which are read by means of a semiconductor laser, development of a recording material forming a color image having an absorption wavelength between 700 nm and 1000 nm is required.
Research into each of color formers and color developers in pursuit of a satisfactory recording material, with attention directed to solubility in oil or water, partition coefficient, pKa, polarity of substituents, position of substituents, change in crystallizability and solubility when used in combination, and the like has been conducted.